1. Field of the Invention
The present invention relates to a data storage device and, in particular, to an arcuate scan tape drive for storing approximately ten or more gigabytes of data on a conventional minicartridge tape.
2. Description of the Related Art
Tape drives are widely used in data processing systems for applications including primary data storage, archival data storage, journaling, and most significantly, as a back-up data storage device to the system's hard drive. Conventional tape drives are designed to transfer data to and from a length of magnetically encoded tape, typically one-quarter inch in width, which tape is transferred between a supply reel and a take-up reel. Currently, most 31/2 inch form factor tape drives utilize a so called "minicartridge" cassette tape for data storage. This type of cartridge is generally described in American National Standard ANSI 3.55-1977. The cartridge measures approximately 9/16" in height by approximately 2-3/8 in width by approximately 3-3/16" in length and carries a data tape approximately 1/4" in width.
While several tape drive designs exist for recording and playing back a data tape, the two most widely used drive technologies up to now have been stationary head tape drives for longitudinal recording and rotary head tape drives for diagonal or "helical" recording.
In longitudinal recording, a tape drive includes a plurality of adjacent stationary heads which are provided to lie across the width of a data tape. As the tape advances past the recording area, each head transfers data to or from a respective track of a plurality of parallel, longitudinally oriented data tracks. A drawback to longitudinal recording is that it is difficult to obtain a high track density across the width of the data tape. Typically in longitudinal recording the read head is slightly narrower than the written data track so that the read head may be located off center of a data track and still lie within the data track. As the number of tracks across the width of the tape increases, the tracks and the read elements become narrower. When there are many narrow tracks, it becomes extremely difficult to find and read the correct track. Consequently, the areal density of a data tape recorded by longitudinal recording must be kept relatively low in comparison to other data storage techniques. While a few longitudinal recording head designs exist having increased areal density capabilities, such systems require complex positioning mechanisms and are relatively expensive.
In comparison to longitudinal recording, rotary head helical recording provides a relatively large areal density. In helical recording, one or more heads are provided around the circumferencial surface of a rotating cylindrical drum. An advancing data tape encounters the rotating drum such that the longitudinal direction of the tape is angled with respect to the plane in which a read/write head on the drum rotates. Thus, as the tape advances and the drum rotates, the drum will record a series of parallel diagonal data tracks on the tape.
With helical recording, as in most data storage technologies, it is necessary to maintain a close physical contact between the read/write head and the storage media. A disadvantage to rotary recording technology is that existing head/tape engagement mechanisms are relatively cumbersome and complex, as well as quite slow to engage and disengage the tape. Consequently, such mechanisms significantly add to the size and expense of the tape drive. Additionally, conventional engagement mechanisms in rotary tape drives involve wrapping the advancing tape around at least a portion of the rotating drum and maintaining a high pressure contact therebetween. This results in a high rate of wear to both the heads and the tape. A still further disadvantage to helical recording is that the recording tape is subject to stretching and shrinking due to wear, humidity and temperature, which stretching or shrinking will distort the diagonal data tracks. Such distortion makes it difficult to accurately align the read/write heads with the data tracks.
Presently in the tape drive industry, as in other data storage technologies, there is a movement toward smaller drive dimensions while at the same time increasing data storage capacity. Existing longitudinal and helical recording technologies have proven inadequate in meeting these demands.
U.S. patent application Ser. No. 07/898,926, filed Jun. 12, 1992 now abandoned, by J. Lemke, which Application was PCT filed on Jun. 10, 1993 and assigned PCT Serial No. PCT/US93/05655, discloses a relatively compact tape drive for recording and playing back approximately 10 gigabytes on a conventional minicartridge. This storage capacity is higher than that previously obtained with either longitudinal or helical recording. The Lemke application discloses a tape drive including a plurality of heads placed on the front circular face of a rotating drum, with the axis of rotation of the rotating drum being perpendicular to and intersecting with the longitudinal axis of the advancing tape. As the tape advances from the right to the left and the drum rotates in a counterclockwise direction, the heads trace arcuately-shaped data tracks substantially transverse to the longitudinal axis of the tape. Arcuate scan recording has been known for some time, but has been disfavored due to at least the lack of effective servoing schemes for accurately maintaining alignment of the heads with the arcuate data tracks.
The head mechanism in the Lemke application includes at least one read, one write and one servoing head mounted on the front face of the rotating drum. The drive further includes a servoing scheme to adjust head/track alignment in two ways in response to servo feedback signals. The first way is to adjust the speed at which the tape advances past the recording area. The second servo adjustment is accomplished by pivotally mounting the head assembly along its length so that the axis of rotation of the head assembly may be tilted upward or downward at the forwardmost face of the drum in response to the servo signals. In this way, the axis of rotation at the front face may be adjusted to align with the centerline of the tape, which tends to stray slightly upward or downward as the tape advances.
The Lemke application discloses two alternative methods of accomplishing the tilting of the head assembly. In one embodiment, coils of electrically conductive windings are situated both above and below a magnetic positioning piece at the end of the pivoting head assembly opposite the rotary head drum. Each coil sets up a magnetic field upon receipt of an electrical current generated in response to a servo signal. The currents in the top and bottom windings exert opposite repulsive forces on the positioning piece. The Application discloses that the electrical currents in the two coils will balance the positioning piece at the proper tilt angle, depending on which coil exerts a greater repulsive force and to what degree it is greater. In an alternative embodiment, there is disclosed a conventional torque motor which exerts a torque on a pin located on the tilt axis of the head assembly, midway along the length of the head assembly. This pin is attached to the head assembly and the tilt angle of the head assembly is determined by the intensity and direction of the current supplied to the torque motor.
In arcuate scan tape drives, such as those disclosed in the Lemke and instant applications, it is imperative that the servo schemes and servo structures be able to quickly and accurately correct the position of the heads to properly align with the arcuate data tracks. A disadvantage to tape drives such as disclosed in the Lemke application is that the structures described above for accomplishing the tilting of the head assembly are unable to correct the tilt position of the head assembly with sufficient quickness or accuracy for proper functioning of the tape drive. This imposes a limit on the amount of data that can be recorded on a given length of tape.
Another reason arcuate scan recording has traditionally been disfavored is because of the relatively poor head/tape interface and tribology which may be achieved with conventional arcuate scan recording systems. As opposed to rigid recording media such as magnetic disks, magnetic recording tape is flexible and, absent some control mechanism, the tape will not maintain a fixed, constant and repeatable position as it travels past the read/write heads in the recording area. A fixed, constant and repeatable head/tape interface is imperative to accurate alignment of the read/write heads with the data tracks, as well as to obtaining a high storage density on the data tape.
A conventional method of obtaining a fixed, constant and repeatable head/tape engagement has been to apply a tension longitudinally to the data tape in the tape cassette. This results in satisfactory tape rigidity across a portion of the width of the tape nearest the centerline of the tape. However, this method is not able to apply a uniform tension across the width of the tape, and as such, the edges of the tape remain relatively slack. Thus, conventional arcuate scan recording systems have not been able to obtain a close head/tape interface near the edges of the tape. While increasing the tension in the data tape will result in a higher degree of rigidity and greater head/tape contact area across the width of the tape, this also results in excessive wear and damage to the head and/or tape in a relatively short period of time.